Aqua Culture

WHAT IS AQUACULTURE

An understanding of the principles of operation of capture and culture fisheries helps to throw light on the definition of aquaculture. The expressions capture and culture fisheries are self-explanatory. In the former, one reaps the aquatic harvest without having to sow, whereas, in the latter, one has to sow the seed, nurse it, tend it, rear it and harvest it when it grows to marketable size.

Examples of capture fisheries are the natural fisheries of the seas, estuaries, rivers, lagoons, large lakes etc.

Culture fisheries are usually carried out in small water bodies which can be manipulated, pre-prepared for stocking; which are often manured and/or fertilized before, during and after stcking; and/or where fish are fed from extraneous sources.

Pen culture, cage culture, culture in running waters, in recirculating systems and in reconditioned water are special types of aquaculture.

All shades of intermediate stages between true capture and culture fisheries exist such as in man-made-lakes, which are stocked extraneously but where no manuring, fertilizing and feeding are generally done. Stocking is often done in large water-bodies such as lagoons and rivers where natural stocks have undergone ‘depletion’.

The principles of management of capture and culture fisheries are very different from each other. In the case of capture fisheries one has to attempt to harvest maximum sustainable yield by regulating fishing effort and mesh after taking into account parameters of population dynamics such as rates of recruitment, natural and fishing mortalities, fish growth and size at which recruitment occurs. Management of capture fisheries requires knowledge of the dynamics of the fish populations under exploitation. The extended exclusive economic zone of 200 miles brings into focus the national and international complexities of regulating the capture fisheries of the seas and the oceans and apportionment of the marine harvest because fish populations do not abide by man-made boundaries.

In the case of culture fisheries, no detailed knowledge of the population dynamics of the cultivated finfish or shell fish is involved. Here, one has to breed, if one technically can, the chosen fish under controlled conditions, if it does not breed naturally, and develop fish husbandry practices so as to be able to formulate economically viable technologies. For effective aquaculture, one has to gain familiarity and control water quality to enhance its biological productivity; one has to understand fish nutrition so as to be able to formulate nutritionally balanced fish diet; one has to delve deep into fish genetics so as to be able to evolve new varieties and strains which bestow commercial advantages to the product in terms of superior growth rate, nutritive value, bonelesness, taste, odour etc.; one has to prevent incidence of fish infections and diseases through prophylatics and therapeutics.

With this background information, a definition of aquaculture can be attempted.

Aquaculture has been defined by the Japanese Resource Council, Science and Technology Agency as under:
“Aquaculture is an industrial process of raising aquatic organisms upto final commercial production within properly partitioned aquatic areas, controlling the environmental factors and administering the life history of the organism positively and it has to be considered as an independent industry from the fisheries hitherto.”
Aquaculture is organised production of a crop in the aquatic medium. The crop may be that of an animal or a plant. Naturally, the organism cultured has to be ordained by nature as aquatic.

Examples are:

Finfish:	Tilapia, carp, trout, milkfish, bait minnow, yellow tail, mullet, cat fish.
Shellfish:	Shrimps, prawns, oysters, mussels, pearl oyster for cultured pearls (eg. Japanese pearl oyster, Pinctada fucata).
Plants:	Water chestnut (Trapa natans). Red alga of Japan, “Norie” (Porphyra). Red alga of Philippines & U.S.A. (Eucheuma) Brown alga of Japan, “Wakame” (Undaria).

During the last decade or so there has been noticeable a global upsurge for aquaculture. Some of the factors which have contributed to the upsurge are:

1. Increased and continuously rising cost of fishing operations due to steep rise of the price of fuel.
2. Fear of reduction in marine fish landings by countries that depend on fishing in the territorical waters of other countries as a result of the new laws of the sea of 200 miles exclusive economic zone.
3. Need, in some countries, for finding alternative and/or additional employment for large numbers of surplus fishermen or under-employed farmers.
4. A persistent demand in most developed countries for high cost species like shrimps and prawns. This has greatly promoted interest in aquaculture in countries that wish to increase their foreign exchange earnings.

The behaviour of one of the world's most productive capture fisheries viz. that of Peruvian anchovy has, I believe, given a hard blow to aquaculture on the one hand, in making scarce and increasing the cost of fishmeal, which is the ingredient of most fish feeds, and, on the other, seeing the helpnessness of man in countrolling natural causes of fluctuations of marine fish yields, has created a desire in him to acquire control on processes of production through aquaculture.

Factors which have been unfavourable to the development of aquaculture are:

1. Shortage of fertilizers in most developing countries and their allocation to agriculture. In this respect, there is a measure of conflict between agriculture and aquaculture.
2. Increasing prices and even the availability of fish meal, which, as stated earlier, is the ingredient of most fish-feeds. This is linked with the Peruvian Anchovy crisis, which, apart from aquaculture, adversely hit agriculture, through scarcity of guano and fertilizer, and poultry industry through scarcity of fish meal. This has led to search for cheaper protein substitutes in fish feeds and spurt of research activity in that direction in different countries.
3. While a general global environmental consciousness has ameliorated aquatic pollution and has thus helped fish culture, aquaculture itself is considered by some as a polluting agent, through release of water containing fish metabolites leading to eutrophication in the recipient waters, which may be a stream or a river or another kind of natural water-body. Discharge regulations which are applicable to aquaculture by authorities in some countries.
   The basic fact is that fishes in general help to keep the aquatic environment clean through exercising biological control of vectors (eg. of water-borne diseases like malaria, filaria etc). Aquaculture water and pond bottom mud often act as fertilizers to agricultural fields. Rarely does aquaculture discharge-water cause pollution.
   Authentic proof it required to establish that aquaculture is a polluter. In whichever case, if it is proved that aquaculture has polluted the environment, the discharge water from aquaculture establishment would need to be treated and rendered innocuous before release.
4. Aquatic pollution, through discharge of agricultural pesticides, domestic wastes, trade effluents and oil spills, has very adversely affected aquaculture. In this respect, there is a measure of conflict between agriculture, especially cultivation of high yielding varieties (HYV) of cereals, and aquaculture. E.g. cases of fish kills in streams and other water-bodies where pesticides fall or where industrial effluents are discharged and adverse effect on oyster beds off Japanese, U.S.A. and French Coasts. The well known cases of oil spills are those of the tankers: Tory Canyon (1967) and Amoco Cadiz (1978).
5. Absence of a constitutional provision for aquaculture as a discreet national activity and legal frame-work for governing its development and administration in most of the countries of the world are standing in the way of entrepreneurs making investment in aquaculture.

Multi-disciplinary and systems characteristics of modern aquaculture need to be especially emphasised in a lecture on definition of aquaculture. Mention has been made earlier of some of the essential components of aquaculture such as water quality control, fish breading, fish genetics, fish nutrition, fish feed formulation, fish pathology, fish parasites and predator control etc. An aquaculturiest has to successfully carry out a whole series of operations before be is able to market his produce.

Complete package of practices have to be developed which involve accomplishment of several steps such as fish multiplication, nursing, tending, and rearing the young, all of which require special food for the larvae and the young fish; then growing the young to marketable size which require special feed again and often intensive feeding for quick growth. The quality of fish feed would naturally depend on the species cultivated. All the above mentioned steps in the practice of aquaculture require rigid water quality control. The cultivated fish has to be saved from the depredations of predators all along its culture. The health of the fish has to be continuously monitored and guarded against infections and infestations which have got to be checked.

The systems approach stands in contrast with disciplinal studies where a scientist take s.up a specific problem and goes deep into it to investigate a certain phe no menon or seeks to establish cause and effect relationship. Even the latter, depending on the nature of the problem, may be multi-disciplinary but it need not always necessarily be. A biochemist, for example, can effectively study fish nutrition and feed components of fish required at different stages of its life but, for successful aquaculture, the whole system involving scores of aspects, some of which have been high-lighted above, have to be worked out.

2. OBJECTIVES OF AQUACULTURE

Having defined aquaculture and mentioned some of the reasons which have contributed to imparting a fillip to aquaculture in recent times, it is proper to state the objectives of aquaculture. These are:

1. Production of protein rich, nutritive, palatable and easily digestible human food benefiting the whole society through plentiful food supplies at low or reasonable cost.
2. Providing new species and strengthening stocks of existing fish in natural and man-made water-bodies through artificial recruitment and transplantation.
3. Production of sportfish and support to recreational fishing.
4. Production of bait-fish for commercial and sport fishery.
5. Production of ornamental fish for aesthetic appeal.
6. Recycling of organic waste of human and livestock origin.
7. Land and aquatic resource utilization: this constitutes the macro-economic point of view benefiting the whole society. It involves (a) maximum resource allocation to aquaculture and its optimal utilization; (b) increasing standard of living by maximising profitability; and (c) creation of production surplus for export (earning foreign exchange especially important to most developing countries).

9. Providing means of sustenance and earning livelihood and monetary profit through commercial and industrial aquaculture. This constitutes the micro-economic point of view benefiting the producer. In the case of small-scale producer, the objective is to maximise income by greatest possible difference between income and production cost and, in the case of large scale producer, by maximising return on investment.
10. Production of industrial fish.

Fish flesh, on the average, contains: moisture and oil, 80%; protein; 15–25%; mineral matter, 1–2%; and other constituents, 1%. Water content is known to vary inversely with fat content.

Need for artificial recruitment has arisen in order to replace or augment stocks decimated by:

1. decline of water quality and destructive fishing (e.g. pollution, poisoning, dynamiting);
2. barrier to migration caused by execution of river valley projects (e.g. anadromous fish) and
3. overfishing.
   From the global view point, the fish which have overwhelmingly dominated artificial recruitment are: i) Oncorhynchus ii) Acipenser iii) Salmo. Artificial recruitment of carp, tilapia and mullet are also important mostly in tropical and subtropical countries.

Oncorhychus and Salmo transplants have contributed maximum to sport and recreational fishing.

Production of livebait e.g. for skipjack tuna (Katsuwonus pelamis) is an example of bait production for commercial fishing. Some potential live-bait species are: Tilapia mossambica, Dorosoma petenense, Engraulis japonicus, Sardinella malanure, several species of mullets and cyprinids.

A wide variety of ornamental fish such as sword tail (Xiphophrus helleri); angel fish (Pterophyllum scale), siamese fighter (Betta splendens), goldfish, and common carp. The last mentioned supports intensive breeding of fancy carps (live jewels) of Japan.

There has come into being fish-cum-livestock culture, in the form of an integrated system especially involving cattle, pigs, ducks and poultry.

Several by-products are obtained from fish. They include fish meal used for animal feeding (in aquaculture an important component of most fish feeds) and as manure; fish flour; fish oil; leather; gelatin and glue from fish skins; imitation pearls; isinglass; adhesives; insulin from fish pancreas; sex hormones from gonads etc.

Production of industrial fish includes production for purposes of reduction to fishmeal or fertilizers. Seaweeds are cultured for marine colloids and pearl oysters for cultured pearls.

3. COMPARATIVE EFFICIENCY OF AQUACULTURE AS A MEANS OF PROTEIN PRODUCTION

While it is recognized that aquaculture provides protein-rich food, it should be pointed out that protein production through aquaculture is much more efficient than production in any other animal production system.

Fish can consume more protein than other animals and can efficiently convert nitrogen in feed into structural proteins in the body. The higher efficiency of nitrogen excretion in fish is another reason for fish to benefit from a bioenergetic point of view. Fish and other aquatic animals have ammonia (NH₃ + NH₄ +) as the major excretory nitrogenous product. For mammals (urea) and birds (uric acid) the excretory products are larger compounds. When proteins are oxidized the endproduct is ammonia, which, in view of its toxicity cannot be allowed to accumulate in body fluids. Whereas in the case of fish ammonia produced can be directly excreted out (mostly through gills) to water, this cannot be so in case of terrestrial animals, and therefore the ammonia produced has to be detoxified by synthesis (energy demanding) of larger molecules such as urea (which is non toxic and can easily dissolve in water and can be excreted in urine in as the case of mammals) and as uric acid in the case birds and reptiles*. The uric acid is a even larger compound than urea:

* Since birds and reptiles are egg laying, the egg cannot accumulate toxic ammonia or carry urea in solution (since this cannot be excreted in urine) and therefore the synthesized uric acid is precipitated and stored as crystals (solid) in a small sac the “allantois” in the eggs of birds and reptiles until hatching.

It is possible that it is this saving in energy (in view ammonia need not be converted into larger compounds, using energy) and also the concomittant enhanced capacity to convert feed nitrogen into tissue protein that the fish is a more efficent producer of protein than the cow or the chicken. Also added to this is the fact that the poikilothermic animals do not expend any energy maintaining a warm body temperature.

When fed on balanced diet under favourable environmental conditions a food conversion ratio (FCR) of 1:1 of wet food eaten to gain in weight (net weight gain to dry feed) has been obtained in fish. The FCR could even be less than unity, but then, it must be remembered that there is difference in level of hydration. For channel catfish and rainbow trout an FCR of 1.0 – 1.25 (weight gain of 1 – 1.25kg for every kg of dry food consumed) has been obtained.

The protein efficiency ratio (weight gain per unit of protein intake) is often higher for fish than for pig, sheep or steers. As pointed out fish are able to utilize high levels of protein in the diet. For reasons linked with those explained already, in poultry almost one half of amino-acids are deaminated and lost for protein synthesis, in weaning pigs two thirds of amino-acids are lost through deamination. It is suggested that the high efficiency of protein synthesis may be related to the high plasma amino-acid level of fish, and also owing to the capacity to excrete NH₃ as explained. It is thus clear that fish are very efficient converters of protein into fish meat.

A comparative study of economics of production in India has shown that fish culture can give 3–4 times profit than the cultivation of wheat, rice and millets. Similarly cost of production of beef, poultry and pork in Hungary shows that fish production costs are the lowest, being 49, 30 and 2 percent less respectively.

4. HISTORY OF AQUACULTURE, ITS PRESENT ORGANISATION AND STATUS

4.1. History of aquaculture

In the historical past, aquaculture remained multilocational and isolated, each location having evolved its own pattern, until in recent times, when with the development of fast means of communication and travel bridging distances in progressively decreasing time, species are being cultured adopting a measure of standardised practices and sites when they are most suited.

The ‘Art’ of aquaculture is very old. The evidence that Egyptians were probably the first in the world to culture fish as far back as 2500 B.C. come from pictorial engravings of an ancient Egyptian tomb showing tilapia being fished out from an artificial pond. The Romans are believed to have reared fish in circular ponds divided into breeding areas. Culture of Chinese carps was sidespread in China in 2000 B.C. Writings in India made in 300 B.C. suggest means of rendering fish poisonous in the Indian sub-continent in times of war. This implies that fish culture prevailed in some Indian reservoirs. Some historical documents compiled in 1127 A.D. describe methods of fattening fish in ponds in India. Culture of Gangetic carps in Bengal in the Indian Sub-continent is of historical origin.

The Chinese carried with them their traditional knowledge of carp culture to the countries they emigrated like Malaysia, Taiwan, Indonesia, Thailand, Cambodia, Vietnam etc. In the Philippines, fish culture has been done in brackishwater ponds for centuries. Eel culture in Japan is also very old.

In Central and occidential Europe, common carp culture developed along with monasteries in the middle ages. Later, with the development of pond fertilization and artificial feeding, carp culture got a new lease of life especially in Central and Oriental Europe. Simultaneously in Europe, salmonid culture began, fillip having been provided by salmon breeding and rearing techniques which were developed by them. Pollution in the aftermath of industrialisation, and hydro-electric development, led to restocking of open waters in Europe. This gave a new texture to development of aquaculture in Europe.

In North America, fish culture has developed from the turn of the century emphasis having been laid on trout for stocking in coldwater and black bass in warm waters.

Except for the referred culture of tilapia in Egypt, the origin of fish culture in Africa is recent. It was only at the end of II world war that efforts were made to introduce and develop fish cultivation. The prize species in Africa is tilapia, which, in recent years, has been extensively transplanted into many warm countries almost round the equator. Tilapia has been referred to as the ‘wonder fish’ of Africa and several attempts to popularise tilapia culture in various African countries did not achieve so much success as expected. In some countries mixed culture of tilapia and catfish Clarias gariepinus) have achieved some success lately; aquaculture prospects and priorities for Africa are now subject to a fresh scrutiny in attempts to make it a successful venture, especially in view of its role in rural development.

Fish culture is only beginning in Latin America and most of the Middle-East. In Israel it has made phenominal progress.

Since World War II, four factors have contributed to rapid development of aquaculture. These are:

1. Facilities of fish transport by modern forms of communication bridging distances by quick transport.
2. Use of polythene bags and fish transported therein under oxygen with addition, when necessary, of transquilizer to water.
3. Artificial propagation of farmed fish (e.g. by hypophysation) and its application to difficult-to-breed fish (e.g. Chinese and Indian carps) and development of hatching techniques to rear eggs and larvae.
4. Availability of feed concentrates and their distribution in pellet form.

The fish which have figured most in inter-regional transplantation are rainbow trout, carp, certain species of tilapia (T. mossambica and T. nilotica) and Chinese carps (Ctenopharyngodon idella and Hypophthalmichthys mollitrix).

Fish culture using some standard methods has, in recent years, got itself extended to many parts of the world. Fish breeding, artificial fertilization and pellet feeding, which at one time were applied to selected species, are now made applicable to many cultured species and, as time advances, more and more species are falling in line, though details vary. With further research in aquaculture, especially on production of fish seed and fish feed technologies, aquaculture in heading towards a quantum jump in years to come.

4.2. Organisation of aquaculture

In China and Socialist countries of Europe which account for a high proportion of present day aquaculture production of the world, fish culture is done in state farms, communes or through cooperative endeavour. In these countries, aquaculture received special attention because of the role of community welfare that aquaculture plays.

In industrially advanced countries, aquaculture is carried out by private sector, i.e. private individuals and companies. In North America, Japan and West Europe, private companies have become increasingly more prominent in the practice of aquaculture.

In developing countries, aquaculture is mostly practiced by small-scale or subsistance level farmers. In these cases, there is heavy dependence on governmental support, including technical and financial assistance. In most cases however, the government is not fully responsible in support of aquaculture. The reason appears to be that aquaculture lacks a firm legal status of its own, it being classified neither as agriculture, nor animal husbandry nor even truly fishing (capture fishery). Aquaculture does not qualify itself for governmental support and incentives given to agriculture and animal husbandry.

On the other hand, the restrictive practices intended for animal-waste disposal are at places indiscrimately applied to aquaculture, sometimes branding it as a polluter.

Most governments include aquaculture under fishery sector despite the productive phase of aquaculture being more skin to agriculture (e.g. manuring and fertilizing practices). For reasons already stated earlier, the positive role aquaculture plays in contributing to national wealth, resource utilization and production of protective protein food, aquaculture is on way to occupying a position of its own in many countries. Once the economic viability of aquaculture in respect of scores of species and multiplicity of systems is fully established every where, as it is bound to happen in due course of time, aquaculture will lead to “aquaplosion” (Jhingran, 1982).

4.3. Characteristics of Aquaculture and its present status

Despite the fact that fish culture is an age-old practice in some regions of the world, it is relatively new as a significant industry in most countries.

Aquaculture is considered to be a labour-intensive, but a high-risk bio-industry. An important characteristic feature of aquaculture is that, depending on its intensification, it can be organised as systems which may be termed as:

a) Extensive	-	Adoption of traditional techniques of aquaculture e.g. dependence on natural productivity and little control over the stocks.
b) Intensive	-	Adoption of full complement of culture techniques including scientific pond design, fertilization, supplemental feeding or only feeding without fertilization; full measure of stock manipulation, disease control, scientific harvesting, high level inputs and high rate of production.
c) Semi-intensive	-	Adoption of mid-level technology, partial dependence on natural productivity, fertilization, supplementary feeding, with stock manipulation, medium level inputs and medium rate of production.

Another characteristic of aquaculture is that it can be organised on the basis of:

1. small scale rural aquaculture (even as one-family-unit).
2. large-scale vertically integrated aquaculture (VIA) which is defined as a centrally mamaged comprehensive system such that all components from input of energy to final level of produce in the market are coordinated and kept in harmony.

Aquaculture is estimated to contribute 10.21 million tons in fish production in 1983.

Group-wise breakdown of the contribution of aquaculture is:

finfish	4.45 million tons
mollusc	3.25 million tons
crustaceans	0.12 million tons
sea weeds	2.39 million tons
	10.21 million tons

Region-wise aquaculture production (million tons) follows the following pattern:

Asia	8.41
Africa	0.05
Latin America	0.22
Europe	1.22
North America	0.31
Total	10.21

From the above it comes out, Asia is the largest producer, contributing about 84% of the world's total aquaculture production. Total aquaculture production for Africa is low and it appears that the reported production has not increased consistently over the past decade. This aspect needs some serious study and we shall refer again to the problem during our present course. One should refer to studies on critical analysis of factors responsible for aquaculture development to understand the problems concerned (see references at the end of this discussion).

5. DIFFERENT KINDS OF AQUACULTURE

As habitats of aquaculture, there are three categories of waters, viz. fresh, salt and brackish. Fresh waters, generally abounding in the inland areas of a country, and the salt water of the seas and oceans, are characteristed by a wide difference in their salinities ranging from nil in the former to nearly 35 ppt in the latter. The difference in salinity within each category of water, fresh and sea, is restricted to rather narrow limits. The salt content of fresh and sea water exercises a very selective influence on the fauna and flora that live in each type of water.

In as far as finfish and shellfish are concerned, the normal residents of each type of water are said to be stenohaline, i.e. they can withstand only a narrow variation in the salinities of their surrounding medium. A carp is an example of stenohaline freshwater fish and a sardine or a mackerel may be cited as examples of stenohaline saltwater fish.

Brackish water normally naturally occurs in estuaries, deltas of rivers, lagoons and backwaters, which everywhere in the world are under tidal regime. In such habitats the salinity of the water fluctuates widely between negligible to 35 ppt, depending on the phase of the tide and volume of fresh water discharged through the river into the sea. The finfish and shellfish that inhabit brackish waters are invariably euryhaline i.e. they form a group of organisms which physiologically withstands wide changes in salinity of the surrounding medium. Stenohaline organisms are devoid of physiological mechanisms to tolerate wide changes of salinity. So, a special type of fauna inhabits the estuarine habitat beyond the sea-end of which live the stenohaline and saltwater forms. Examples of euryhaline fish are a mullet (Mugil cephalus) and mud-skipper, Periophthalmus* and those of crustaceans are several species of penaeids (e.g. Penaeus monodon)* and crab (e.g. Scylla serrata*).

The capacity of the residents of an estuary to tolerate a wide range of salinity that prevails there is by virtue of a dynamic physiological process of osmoregulation in which the gills, the kidneys, the skin and the buccal cavity lining play significant roles.

* Periophthalmus koelereuteri, Penaeus notialis and the crab, Callinectes are corresponding species which we encounter in ARAC fish farm at Buguma.

There are finfish and shellfish which spend different phases of their lives in sea, estuaries and freshwater streams. These forms transcend the salinity barrier by their osmoregulation. Such animals are either anadromous or katadromous. Anadromous fish, as exemplified by salmon or Acipencer or shad, are those that bread naturally in freshwater streams but spend the middle years of their lives in the sea. Katadromous forms, as exemplified by the eel, display the opposite kind of life cycle. These animals breed in the sea and spend the middle years of their lives in fresh water streams.

There are forms which restrict their migration between fresh water sections of the river and the estuary. Several species of palaeomonid prawns (Macrobrachium rosenbergi; M. vollenhovenii) are examples of shellfish which undergo such a life cycle. These forms breed in estuaries but spend the mid-years of their live in fresh waters. Then, there are forms which migrate back and forth between the estuary or a lagoon and the sea in different phases of their lives. A mullet (e.g. Mugil cephalus) or a shrimp (e.g. Penaeus mododon, P. notialis) are examples of finfish and shellfish which show such a pattern of migration. These forms breed in the sea but spend part of their juvenile and adult lives in the estuary where they form a sizeable fishery.

Apart from salinity of the water, its temperature exercises a selective influence on fish that thrive there e.g. warmwater fish as contrasted with temperate or coldwater fish. Even in tropical countries, a river may have and usually does have, a coldwater section in its upper reaches and a warmwater section in its middle and lower reaches. In temperate countries and in the upper reaches of tropical countries (e.g. at high altitudes), coldwater fish (e.g. trout, loach etc.) live. Then warm waters have their distinctive fish fauna (e.g. scores of species of carps and catfishes and several species of murrels etc.).

Notwithstanding the fact that the capacity of water to dissolve oxygen (DO) is negatively correlated with temperature, the oxygen content of water at a given temperature can vary a great deal depending on turbulance, photosynthesis and BOD. Do of water exercises a selective influence on quality of fish life. In water of low oxygen content, air-breathing fish thrive best e.g. Clarias*, snakeheads** etc. Fish that are used to living in well-oxygenated water e.g. trout, do not thrive in waters of low oxygen content.

Notwithstanding differences in the physioco-chemical characteristics of its habitats (viz. fresh water, brackish water and sea water) aquaculture systems are of several kinds. Most of the systems are highly variable in mannitude and intensity ranging to serve as one-family units or large scale commercial enterprises. The different kinds of aquaculture are:

1. Static water ponds.
2. Running water culture.
3. Culture in recirculating systems: in reconditioned water and in closed systems.
4. Culture in rice fields.
5. Aquaculture in raceways, cages pens and enclosures
6. Finfish-culture cum livestock rearing.
7. Hanging, ‘on-bottom’ and stick methods of oyster culture.

* The snake head Ophiocephalus obscurus is the common form in West Africa.

^** C. gariepinus (=lazera) is most important commercially, among about 10 species of Clarias which occur in the Rivers State (see J. Akiri, 1987, studies on Clarias species in Rivers State. Ph.D. thesis, Rivers State University of Science & Technology).

Based on the number of species that are cultured in a system aquaculture may be classified into: (a) monoculture and (b) polyculture.

5.1. Pond Culture

5.1.1 Static freshwater ponds

Ordinary fresh water fish culture ponds are still-water ponds. They vary a great deal in waterspread area and depth. Some are seasonal and some perennial. The ponds may be rainfed (also called sky ponds) and/or may have inlet and outlet systems. The water supply may be from a stream or a canal or from an underground source such as wells, tubewells etc. The water retentivity of the ponds depends on soil composition of the pond bottom and subsoil water level. The natural biological productivity of such ponds depends on soil and water qualities. Homestead ponds are usually small and shallow. Commercial freshwater ponds have to have an assured water supply and inlet and drainage systems. In organised aquaculture, the carrying capacity of still-water ponds is enhanced by manuring and/or fertilizing and exercising water quality control. Fish are also fed from an extraneous source for obtaining fast growth.

Science of freshwater pond fish-culture has made great strides in recent years and there is a fast advancing frontier of knowledge on every aspect of pond culture starting from farm designing and construction upto production of marketable fish of a wide variety of cultured fresh water species of finfish and shellfish. Examples are: carp culture systems in India, China, Israel, Germany, etc; catfish culture in U.S.A.

There is considerable competition with agriculture and other land-use agencies in this system of aquaculture and its success would, by and large, depend on comparative economics of land use. But much also depends on national policies on land use and the encouragement government gives to aquaculture as a means of producing fish protein.

5.1.2 Brackishwater ponds

Not only are the species different from those cultured in freshwater ponds but the principle of operation of brackishwater ponds is different from those of freshwater ponds. Here the pond or the farm is essentially located on a tidal creek or stream and there is a system of sluices to control the ingress and egress of water into and from the ponds. Examples are: Milkfish farms in Philippines, Taiwan, Indonesia etc. Brackish water fish farming is a fast growing science. Here also there is competition with other land use agencies, especially forestry, but the extent of competition with agriculture is relatively less because coastal land is generally not suitable for agriculture. The ARAC farm at Buguma is tidally fed and the salinity range is 5 – 21 ppt.

Mariculture:

Mariculture is aquaculture in the saltwater of the sea. It may be in seas, bays, bayes, sounds etc. e.g. traditional mariculture in inshore and offshore waters by a large number of countries notably, U.S.A., France, Spain, Japan etc. Mariculture of finfish in cages is relatively recent. Though a new development, it has assumed considerable importance and has great potential e.g. mariculture of several species of salmonids; Salmo salar, Oncorhynchus spp; of yellow tail, Seriola quinaueradiata; of red seabream (Pagrus major) etc.

5.2. Running water culture

In Japan, at places where there is abundant supply of water, common carp is cultured in running water ponds. The most intensive common carp is cultured in running water ponds. A very high common carp production rate of 980 t/ha has been achieved at the Tanka Running water fish farm in Japan where there is plentiful supply of running water of high dissolved oxygen content and optimum range of temperature for feeding. Running water culture of common carp is done in a small way in Europe, Indonesia and Thailand.

5.3. Culture in recirculatory systems

This system is comparable to running water culture system except that in the latter, water goes waste whereas here the same water is reused. In this system, water is filtered continuously and recirculated, often after aeration, to the fish pond. The filtering element is a biological filter comprising 3 – 4 cm diameter pebbles, or honey-comb synthetic strips, designed to arrest faecal matter and to denitrify catabolic wastes through bacterial action. The Motokawa Fish farm in Japan is well known for carp production in recirculatory filtering ponds. This system has been tried experimentally for carp fry rearing at the Central Inland Fisheries Research Institute, Barrackpore (W.B) India, with commendable results. There are several new developments in reusing water for fish culture, given in two volumes, by Tiews (1981). See also EIFAC (1986).

A recirculatory system is sometimes classified as a system of waste-water aquaculture and reducing biological oxygen demand (BOD) of the waste water. Several mechanisms of handling waste-water exist viz.

1. pretreatment of waste water e.g. cascading through a series of ponds: air system etc;
2. dilution of waste water;
3. pretreatment and dilution;
4. no treatment of waste water.

Benefits lie in:

1. increase in fish yields through increase in natural fish food, 5000 kg/ha at Aquaculture Research Station, Dor Israel from fluid cowshed manure; sewage fed system in Bengal and Tamil Nadu, India - 15,000 kg/ha/year.
2. direct use of solid organic matter in natural waters by phytoplankton and zooplankton.

Restraints in wastewater fish culture systems lie in:

1. Do level in ponds;
2. toxic material in wastewater;
3. tastes and odour in fish;
4. parasites and diseases;
5. public health problems (Salmonella, Shigella) of other Enterbacteriacae;
6. pond effluent standards; and
7. (7) public acceptance.

These lead to problems of fish pond management i.e. acquiring understanding of physioco-chemical dynamics of pond in relation to physiological requirements of cultured species. Polyculture system need to be encouraged where productivity is based on natural foods. e.g. ecological niche approach in the polyculture of Chinese and Indian carps in India.

In reconditioned water:

Water reconditioning in aquaculture is necessary where there is inadequate water supply for the fish culture programme or where water quality requirements are such that reconditioning is indicated.

The different fish rearing systems using reconditioned water are:

1. simple flow-through (single pass) system i.e. ample water-supply of appropriate quality
   
2. single pass system with pre and post-treatment:

i.e. Water supply 

3. a system re-using water in fish rearing unit:
   

The pre-treatment processes used are:

(1) sedimentation; (2) screening; (3) shocking to kill aquatic life; (4) filtration; (5) sterilization; (6) aeration; (7) degassing (nitrogen, CO₂ removal) (8) heating or cooling if necessary (9) pH control.

The reconditioning processes are:

1. sedimentation;
2. mechanical filtration;
3. biological filtration;
4. extended aeration;
5. activated sludge;
6. pH control;
7. heating or cooling as the need be;
8. sterilization;
9. de-gassing and
10. ion exchange.

Post-treatment processes are:

(1) aeration; (2) sedimentation; (3) filtration; (4) disinfection; (5) activated sludge; (6) lagooning; (7) digestion or equivalent; (8) coagulation; (9) absorption taste or odour removal by activated carbon.

These systems are mostly applicable to sophisticated and intense aquaculture.

In Closed System:

Fresh water model developed at Ahrenbury in Germany is a 50 m³ in₃ circuit for mirror carp, Cyprinus carpio, 6 m³ in fish tanks and 44m³ in purification unit. Temperature kept constant at 223°C and quantity of food (g of food/fish/day) has been kept equal. Rate of flow: 25 m³/hour; maximum carrying capacity, determined experimentally being 1.5t of fish and ratio of water volume to fish weight 30:1. Closed circuits have limited carrying capacity and when the capacity is exceeded, the system may break down.

The highest production, an annual yield of 8.6t of carp is obtained using semi-monthly rather than monthly and 1.5 monthly stocking sequence, fish are harvested when 500 g in weight (rather than 1000, 1500, or 2000 g) and fed on fish having raw protein content of 36% (rather than 47%).

The cost of feeds are the main operating costs when using the system. The decrease in income is proportionately more rapid than decrease in production when the size of fish harvested increase and stocking sequence are prolonged. The most economical has been found to be when heat is derived from heat exchanges from industrial cooling water rather than when centrally heated or when diverting one circuit out of 12 to fingerlings production i.e. all circuits producing 500 g fish for the market. As indicated earlier many such trials have been done (see Tiews, 1981).

5.4 Culture in Rice Rields

Culturing fish and growing rice together in the same paddy fields is an old practice in Asia and the Far East. Interest in producing rice and fish together had declined in recent years because of use of fish-toxic pesticides required to protect high yielding varieties (HYV) of rice introduced as part of green revolution in Asia. Now, newer HYV of rice strains are being developed with inbred resistance to insects and insect-transmitted diseases which decrease the need for pesticide protection or growing rice. Four trials conducted in Philippines on Tilapia mosambica and Cyprinus carpio stocking have resulted in standing crops of fish in paddy fields averaging 69 – 288 kg/ha at harvest time. More developments in rizi-pisciculture are described in the ICLARM Conference Proceedings on “Integrated Aquaculture - Agriculture farming systems (Pullin & Shehadeh, 1980).

5.5 Aquaculture in Raceways: Cages, Pens and Enclosures

Marine aquaculture farms may be located at six possible sites viz. either on the shore with pumped sea-water supply; in the intertidal zone; in the sub-littoral zone, or offshore with surface floating, mid-water floating or seabed cages. The first three are enclosures and the last three cages. The enclosures in Europe have by and large stemmed from those set up for ‘yellow tail’ (Seriola quinqueradiata) farming in Japan. e.g. of inter-tidal enclosure:

1. Adoike near Takasu in Inland sea in Japan;
2. Ardtoe in Great Britain. Since concrete seawalls or stone-pitched anbankments are expensive, few intertidal enclosures are now built.

5.5.1 Rigid structures

A large number of rigid net enclosures have been built in Inland sea in Japan in recent years, but not all are successful: because of poor siting providing less circulation, others fouled by marine organisms which restrict circulation. More modern successful net enclosures have been positioned after hydrographic surveys to insure sufficient water exchange and research on building material. (Most suitable: galvanised “chain-link” and galvanised “weldmesh”) e.g.

1. yellow tail 3.5 ha farm at Sakaide in Seto Inland Sea, Japan.
2. Faery Isles, Lake sween, Scotland.
3. Bamboo barricades for milkfish farming in Laguna de Bay, Philippines.
4. Sub-littoral enclosures for salmon farming in Norway e.g. Flogoykjolpo (1.2 ha) and Volokjolpo (3.5 ha) near Movik, west of Bergen (Farming potential os salmon 1000 t.)
5. Sub-littoral enclosure built in 1974 at Loch Moidart (40 m³ capacity) (tidal range of 5m).

5.5.2 Flexible Structures

Buoyed fish net enclosure but resting on bottom e.g. 10 ha trial enclosure in Laguna de Bay, Philippines.

5.5.3 Floating Fish Cages

Most important development of the decade is aquafarming. Its merits lie on:

1. can be used where seabed is unsuitable for shellfish.
2. being off bottom, predators can be controlled more easily.
3. can be towed out of danger if threatened by pollution.

5.5.4 Cages with Rigid Framework

e.g.

(a) 8m diameter 6m deep	having galvanised steel collar, galvanised chain link bag net for yellow-tail farming in Japan.
(b) 14m diameter 7m deep
(c) cages at Loch Ailort	for Salmon farming in Scotland provided with rigid collars and cat-walks for inspection of fish.
(d) cages at Loch Moidart (6m × 4 × 3. 1m)
(f) 50 × 12m Pacific Salmon cage at Reservation Bay near Anacortes, Washington U.S.A.

5.5.4.1 Cages with Flexible Framework

e.g.

1. 55m diameter and 25m deep chum salmon (Oncorynchus keta) in Lake Saroma at Hokkaido, Japan. Here netting material can be changed even when fish are in stock.
2. Midwater fish cage of the Hiketa Fishermen's Cooperative Association of Japan in Inland sea: 9m sq. × 8.25m deep with a buoyed feeding neck. These are grouped in 10– 12 cages. They can be raised in calm sea and lowered in rough sea.
3. Domsea Farm cage of Puget Sound, Washington U.S.A. 15.2 × 15.2 × 7.6m cage.
4. Floating fish cage of Kampuchea and Southern Vietnam.

Several new developments in this area and allied aquaculture are described and reported in British monthly, “Fish farming International”.

5.5.4.2 Merits of Cage Culture

Advanced type of aquaculture having scores of advantages over pond culture.

1. 10 – 12 times higher yields than pond culture for comparable inputs and area;
2. Prevents loss of stock due to flooding;
3. No question of seapage and evaporation losses;
4. No need for water replacement;
5. No problem of pond excavation and dependence on soil characterics;
6. Avoids proximity of agricultural areas hence reduces hazards of pesticide contamination;
7. Can be conveniently located near urban markets avoiding the need for fish preservation and transportation;
8. Eliminates competition with agriculture and other land uses;
9. Affords easy control of fish reproduction in Tilapia sp;
10. Complete harvest of fish is effected;
11. Optimum utilization of artificial food;
12. Reduced fish handling;
13. Initial investme nt relatively small.

5.5.4.3 Limitations of Cage Culture

They are relatively few. They are:

1. Difficult to apply when water is rough;
2. High dependence on artificial feeding. High quality feed fesirable especially in respect of protein, vitamins and minerals. Feed losses are possible through cage walls.
3. At times interferes with natural fish populations round cage.
4. Risk of theft is increased.

In view of these, it is reasonable to consider cage culture practice as one which will prevail in future years. Research on this system deserves to be encouraged.

5.6. Finfish Culture-cum-Livestock Rearing

Commercial scale fish-cum-duck culture is practiced in Central European countries such as Czeckslovakia, East Germany, Hungary and Poland, as well as in Taiwan Province of China. FAO has organised this system in C.A.R., Zambia and Ivory Coast and also in Nepal recently. In this system of culture, fish pond water surface maintains brood stock of ducks, rear one-day-old ducklings as well as 14–21 day-old advanced ducklings. This is a synergic system of mutual benefit to each organism cultured: duck droppings manuring the pond, duck foraging consuming a variety of unwanted biota for fishculture such as tadpoles, frogs, mosquito and dragonfly larvae, molluscs, aquatic weeds etc. One duck produces about 6kg of droppings in 30 – 40 days in a pond and 100kg of duck manure may increase fish flesh to the extent of 4 –6kg. 300 ducks led to an increase by 100kg of carp in East Germany. In Taiwan fish-cum-duck-culture produces 3500kg/ha of fish.

Both poultry droppings and pig excreta are used to manure fish ponds. To save transport costs poultry pen and pig sties are advised to be located at or near fish pond site. A fat pig produces, on the average 1.6 – 1.8t of manure (including urine) per year and fresh manure of 15 – 25 pigs can be used in a one hectare pond. Hungary has developed carbon-manuring technique in early fifties with ducks acting as carbon-manuring machines. In Hungary 30 – 60/ha/100 days of manure is spread. Recently in India polyculture of Chinese and Indian carps in a pig manure fertilized pond led to nearly 7.5 tons/ha/yr production of fish. In C.A.R., production of 10 – 15 tons/ha/yr has been achieved. See Pullin and Shehadeh (1981) for several other experiments on integrated agriculture-aquaculture systems.

5.7. Monoculture

Monoculture, as the name implies, in the culture of a single species of an organism in a culture system of any intensity, be it in any type of water, fresh, brackish or salt.

e.g. Fresh water

Common carp in East Germany

Common carp in Japan

Tilapia nilotica in several countries of Africa
Rainbow trout (Salmon gairdneri) culture in several countries.
Channel catfish (Ictalurus punctatus) in U.S.A.
Catfish, Clarias gariepinus in Africa.

Brackish water

Milkfish, Chanos chanos in the Philippines.
Mullet culture in several countries.

Seawater

Yellowtail, Seriola quinqueradiata in Japan.
Kuruma shrimp, Peneaus japonicus
Nori: Porphyra sp. in Japan
Scallop (Patinopecten yessoesin) in Japan
Red seabream (Pagrus major) in Japan
Pacific salmon (Oncorhynchus spp) in Nort America
Eel (Anguilla spp) in Japan.

Feeding with species spefific feed is the main basis for monoculture in the case of finfish.

5.8 Polyculture

Polyculture, as the name implies, is the culture of several species in the same waterbody. The culture system generally depends on natural food of a waterbody sometime augmented artificially by fertilization and/or by supplementary feeding. If artificial food is given it is a common food acceptable to all or most species that are cultured.

e.g. Fresh water

1. Polyculture of Clarias gariepinus and tilapias in Africa.
2. Polyculture of several species of Chinese carps in China, Taiwan etc.
3. Polyculture of several Indian major carp species in India.
4. Polyculture in Indian major carps, Chinese carps and other fish in India (called composite fish culture in India).

Brackish water

1. Milkfish and shrimp culture in Philippines and Indonesia.
2. Mullet and shrimp culture in Israel. In systems where production depends on natural fish pond zonation i.e. ecological niches assume great importance.

5.9. Hanging, ‘on bottom’ and stick methods of oyster culture

In the hanging method, oysters as they grow, are suspended from rafts, long-lines or racks.

Raft method is used in protected areas as in the Seto Inland sea of Japan.

The long-line system has horizontal lines attached to wooden barrels or metal drums at or near the surface from which strings of seed oysters are suspended. The long-line system is used in offshore grounds. The system can withstand rough seas which might destroy rafts.

The structures in the rack method consists of vertical poles or posts driven into bottom which support horizontal poles. Strings of seed oyster are tied to horizontal poles such that they do not touch the bottom. The trend of rack method is downward because of coastal pollution.

In the sowing method, oysters are directly placed on the bottom.

In the stick method, seed oysters are attached to wooden sticks riven into bottom in the intertidal zone. In both stick & “on bottom” method, crawling predators take a toll of oysters.

Raft and long line methods are most productive as they minimise losses by predation and maximise production. U.S.A., Japan, Republic of Korea, France and Mexico are some of the major oyster producing countries. U.S.A. and France largely use ‘on bottom’ method. The traditional species in France has been Ostrea edulis and Crassostrea angulata but in recent years, heavy mortalities have occurred and France imported Crassostrea gigas from Japan, Canada and U.S.A. to circumvent the problem. In West Africa, including Nigeria, C. gasar, is being tested for mass scale adoption of aquaculture.

6. COST - BENEFIT OF CERTAIN AQUACULTURE SYSTEMS

It has been estimated that on global basis, 75% of aquaculture production is from finfish culture in ponds utilizing about 90% of area used for aquaculture. Seed, fertilizer, feed labour, marketing and interest are the major items of expenditure in pond culture operation. The economic viability of aquaculture can be improved by: (1) increasing per ha yield; (2) reducing cost of production and (3) procuring better price for producer by qualitative improvement of the produce and creating better marketing facilities, strategy and channels.